The present invention relates generally to magnetic random access memory devices, and, more particularly, to a method and structure for selecting the anisotropy axis angle of an MRAM device for reduced power consumption.
Magnetic (or magneto-resistive) random access memory (MRAM) is a non-volatile random access memory technology that could potentially replace the dynamic random access memory (DRAM) as the standard memory for computing devices. The use of MRAM as a non-volatile RAM will eventually allow for “instant on” systems that come to life as soon as the system is turned on, thus saving the amount of time needed for a conventional PC, for example, to transfer boot data from a hard disk drive to volatile DRAM during system power up.
A magnetic memory element (also referred to as a tunneling magneto-resistive, or TMR device) includes a structure having ferromagnetic layers separated by a non-magnetic layer (barrier), and arranged into a magnetic tunnel junction (MTJ). Digital information is stored and represented in the memory element as directions of magnetization vectors in the magnetic layers. More specifically, the magnetic moment of one magnetic layer (also referred to as a reference layer) is fixed or pinned, while the magnetic moment of the other magnetic layer (also referred to as a “free” layer) may be switched between the same direction and the opposite direction with respect to the fixed magnetization direction of the reference layer. The orientations of the magnetic moment of the free layer are also known as “parallel” and “antiparallel” states, wherein a parallel state refers to the same magnetic alignment of the free and reference layers, while an antiparallel state refers to opposing magnetic alignments there between.
Depending upon the magnetic state of the free layer (parallel or antiparallel), the magnetic memory element exhibits two different resistance values in response to a voltage applied across the tunnel junction barrier. The particular resistance of the TMR device thus reflects the magnetization state of the free layer, wherein resistance is “low” when the magnetization is parallel, and “high” when the magnetization is antiparallel. Accordingly, a detection of changes in resistance allows a MRAM device to provide information stored in the magnetic memory element (i.e., a read operation). There are different methods for writing a MRAM cell; for example, a Stoner-Wohlfarth astroid MRAM cell is written to through the application of a bidirectional current in a particular direction, in order to magnetically align the free layer in a parallel or antiparallel state. The free layer is fabricated to have a preferred axis for the direction of magnetization called the “easy axis” (EA), and is typically set by a combination of intrinsic anisotropy, strain induced anisotropy and shape anisotropy of the MTJ.
When a sufficiently large current is passed through both a wordline and a bitline of the MRAM, the combined fields of these currents at the intersection of the write and bit lines will rotate the magnetization of the free layer of the particular MTJ located at the intersection of the energized write and bit lines. The current levels are selected such that the combined fields exceed the switching threshold of the free layer. For a Stoner-Wohlfarth astroid MRAM structure, the EA is aligned with the orientation of either the bitline or the wordline.
As the lateral dimension of an MRAM device decreases, several problems can occur. First, the switching field increases for a given shape and film thickness, thus requiring a larger magnetic field for switching. Second, the total switching volume is reduced such that the energy barrier for reversal also decreases, wherein the energy barrier refers to the amount of energy needed to switch the magnetic moment vector from one state to the other. The energy barrier determines the data retention and error rate of the MRAM device, and thus unintended reversals can occur due to thermofluctuations if the barrier is too small. Furthermore, with a small energy barrier it becomes extremely difficult to selectively switch a single MRAM device in an array without inadvertently switching other MRAM devices. Thirdly, because the switching field is produced by shape, the switching field becomes more sensitive to shape variations as the MRAM devices decreases in size.
In this regard, there has been introduced an MRAM device in which the free layer of ferromagnetic material includes multiple (e.g., two) ferromagnetic layers. Due to magnetostatic coupling, the magnetic moments of the two ferromagnetic layers are antiparallel to one another such that there is a net resultant magnetic moment oriented in the anisotropy easy axis. This configuration allows for a different method of writing that has improved selectivity. More specifically, the writing method relies on a “spinflop” phenomenon that lowers the total magnetic energy in an applied field by rotating the magnetic moment vectors of the ferromagnetic layers such that they are nominally orthogonal to the applied field direction but still predominately anti-parallel to one another. A rotation, or flop, in combination with a small deflection of each ferromagnetic magnetic moment vector in the direction of the applied field accounts for the decrease in total magnetic energy. Current waveforms applied to the wordline and bitline in a timed sequence cause a magnetic field flux to rotate the effective magnetic moment of the device by approximately 180 degrees.
In the rotational method of MRAM writing, the device is constructed such that the magnetic anisotropy axis is at a 45° angle with respect to the orientation of the word and bitlines. From a power consumption standpoint, a 45° angle orientation of the anisotropy axis is optimal only for architectures in which the field generation capability of the word and bitlines are essentially equivalent to one another. However, in real world MRAM architectures, it is often the case that the field generating efficiency of the word and bitlines are not equivalent. For example, there may be variations in the resistance of the lines and/or variations of the position of the lines relative to the MTJ. In addition, magnetic liners may be used to enhance the writing efficiency of one set of lines as opposed to the other. Moreover, for architectures where an entire line of bits in an array are to be written simultaneously, it may be more advantageous to apply a relatively large current to a wordline and to reduce the amount of current applied to the several bitlines.
Accordingly, it would be desirable to be able to determine a customized anisotropy axis angle for a given MRAM architecture that provides a reduction in power consumption, but that still maintains a threshold activation energy such that the MRAM device maintains selectivity.